A number of spinal fixation systems feature a fixation rod that is anchored by two or more bone screws. To interconnect the fixation rod with the bone screws, each bone screw incorporates some form of cap or “receiver body” that is either integral with the screw, or surrounds a head portion on the screw. The receiver bodies include some form of opening, such as an open channel, to receive the fixation rod. Surgeons must apply a significant amount of force to advance or reduce a fixation rod into a proper position in the receiver body. In addition, surgeons must apply a significant amount of force to securely fix the rod in the channel within the receiver body. These forces result in various stresses on the receiver body.